1. Field of the Invention.
The present invention relates to methods and compositions for preventing the loss of high density completion fluids used in completing wells.
2. Description of the Prior Art.
In drilling a well bore penetrating one or more subterranean formations, a drilling fluid is generally circulated through the well bore to provide hydrostatic pressure in the well bore, remove cuttings therefrom, lubricate the drill bit, etc. Most drilling fluids are comprised of water containing a viscosity increasing material and a weighting material such as barite for increasing the density thereof. A variety of additives are also commonly included in drilling fluids to impart desired properties thereto such as low fluid loss and the like.
After the well bore penetrates a permeable subterranean formation containing hydrocarbons, casing is often cemented in the well bore and perforations are formed therein which extend from the well bore through the casing and cement into the hydrocarbon containing formation. Thereafter additional well completion operations can be carried out in the well bore such as the installation of a prepacked screen or other screening device, the formation of a gravel pack, etc.
In order to prevent damage to the permeable hydrocarbon containing formation, prior to perforating the formation the drilling fluid in the well bore is often replaced with a completion fluid comprised of a non-damaging aqueous salt solution which does not contain insoluble solids. When the producing formation penetrated by the well bore contains fluids under high pressure, the completion fluid utilized must have a very high density, i.e., a density up to about 21.5 pounds per gallon to prevent the pressurized fluids from blowing out the well bore. Typically, the completion fluid is an aqueous solution of one or more salts such as potassium chloride, sodium chloride, calcium chloride, sodium bromide, calcium bromide, zinc chloride and zinc bromide. The cost of a high density aqueous solution of such a salt or salts is very high, and as a result, after being used the completion fluid is recovered, filtered and reused. However, during the use of a high density completion fluid in a well being completed, portions of the completion fluid can and very often are lost into the permeable formation penetrated by the well bore. While such fluid loss has been reduced through the use of various fluid loss control additives known to those skilled in the art, portions of the completion fluid are still often lost, particularly during gravel packing and other similar procedures.
Thus, there is a need for methods and compositions for preventing the loss of high density well completion fluids.